Surface acoustic wave device, electronic apparatus, and sensor apparatus

ABSTRACT

A SAW device has an IDT which is provided in the principal surface of a quartz crystal substrate having Euler angles (−1.5°≦φ≦1.5°, 117°≦θ≦142°, |ψ|≠90°×n (where n=0, 1, 2, 3)) and excites a Rayleigh wave (wavelength: λ) in a stopband upper end mode. Inter-electrode-finger grooves are recessed between the electrode fingers of the IDT. The depth G of the inter-electrode-finger grooves is 0.01λ≦G≦0.07λ, and an electrode finger thickness H and an IDT line occupancy η satisfy a predetermined relationship. Thus, a frequency-temperature characteristic constantly has an inflection point between a maximum value and a minimum value in an operation temperature range, thereby suppressing a fluctuation in an inflection-point temperature due to a manufacturing variation in the IDT line occupancy η.

TECHNICAL FIELD

The present invention relates to a surface acoustic wave device, such asa resonator or an oscillator using a surface acoustic wave (SAW), and anelectronic apparatus and a sensor apparatus including the same.

RELATED ART

SAW devices are widely used in electronic apparatuses, such as a mobilephone, a hard disk, a personal computer, a receiver tuner of BS and CSbroadcasts, an apparatus which processes a high-frequency signal or anoptical signal propagating through a coaxial cable or an optical cable,a server network apparatus which requires a high-frequency andhigh-precision clock (low jitter and low phase noise) in a widetemperature range, and a wireless communication apparatus, or varioussensor apparatuses, such as a pressure sensor, an acceleration sensor,and a rotational speed sensor. In these apparatuses and devices, inparticular, with the realization of a high-frequency reference clock dueto recent high-speed performance of information communication or thereduction in the size of the apparatus casing, there is an increasinginfluence of heat generation inside the apparatus. For this reason, withregard to an electronic device which is mounted in the apparatus,expansion or high-precision performance of an operation temperaturerange is required. A stable operation is required over a long period inan environment in which there is a severe change in the temperature froma low temperature to a high temperature, like a wireless base stationoutdoors.

In general, in a SAW device, such as a SAW resonator, a change in thefrequency-temperature characteristic is significantly influenced by theSAW stopband, the cut angle of a quartz crystal substrate to be used,the form of an IDT (interdigital transducer) formed on the substrate, orthe like. For example, a reflection inverting-type SAW converter issuggested in which an IDT having a unit segment repeatedly arranged on apiezoelectric substrate is provided, the unit segment having threeelectrode fingers per SAW wavelength, and the upper mode and lower modeof the SAW stopband are excited (for example, see Japanese Patent No.3266846). If a SAW filter is constituted by the reflectioninverting-type SAW converter, it is possible to realize a highattenuation amount in a blocked band on a high-frequency band side neara passband.

A reflection inverting-type SAW converter is known in which a so-calledST cut quartz crystal substrate having Euler angles (φ, θ, ψ)=(0°, 123°,0°) is used (for example, see JP-A-2002-100959). JP-A-2002-100959describes that the resonance of the upper end of the stopband can beexcited, and the frequency-temperature characteristic is improvedcompared to a case where the resonance of the lower end of the stopbandis used. It is reported that the upper end of the SAW stopband has asatisfactory frequency-temperature characteristic compared to the lowerend of the stopband (for example, see JP-A-2006-148622,JP-A-2007-208871, JP-A-2007-267033, and JP-A-2007-300287).

In particular, JP-A-2006-148622 and JP-A-2007-208871 describe atechnique which adjusts the cut angle of the quartz crystal substrateand thickens the standardized thickness (H/λ) of an IDT electrode toabout 0.1 so as to obtain a satisfactory frequency-temperaturecharacteristic in a SAW device using a Rayleigh wave. A SAW resonatordescribed in JP-A-2006-148622 has a single-type IDT electrode in which aunit segment having two electrode fingers per SAW wavelength isrepeatedly arranged on a quartz crystal substrate having Euler angles(φ, θ, ψ)=(φ=0°, 0≦θ≦180°, 0°<|ψ|<90°). Thus, the Rayleigh wave isexcited in the stopband upper limit mode, thereby realizinghigh-frequency performance and a satisfactory frequency-temperaturecharacteristic of a SAW resonator.

JP-A-2007-208871 describes a technique which, in a SAW device which hasthe single-type IDT electrode, sets a quartz crystal substrate at Eulerangles (φ, θ, ψ)=(φ=0°, 110°≦θ≦140°, 38°≦|ψ|≦44°), and sets therelationship between the standardized electrode thickness (H/λ) and thestandardized electrode width η(=d/P) defined by the thickness H of theIDT electrode, the width d of an electrode finger in the IDT electrode,the pitch P between electrode fingers in the IDT electrode, and the SAWwavelength λ as follows.H/λ≧0.1796η³−0.4303η²+0.2071η+0.0682

Thus, it is possible to strongly excite the Rayleigh wave in thestopband upper limit mode.

JP-A-2007-267033 describes a SAW element in which a single-type IDTelectrode is arranged on a quartz crystal substrate having Euler angles(φ, θ, ψ)=(0°, θ, 9°<|ψ|<46°), preferably, (0°, 95°<θ<155°,33°<|ψ|<46°), and the standardized electrode thickness (H/λ) is0.045≦H/λ≦0.085. Thus, the Rayleigh wave is excited in the stopbandupper limit mode, thereby realizing a satisfactory frequency-temperaturecharacteristic.

JP-A-2007-300287 describes a SAW element in which the single-type IDTelectrode is arranged on an in-plane rotation ST cut quartz crystalsubstrate having Euler angles (φ, θ, ψ)=(0°, 123°, 43.2°), and thestandardized electrode thickness (H/λ) is H/λ=0.06, so-called 6% λ,thereby exciting the Rayleigh wave in the stopband upper limit mode. TheSAW element sets the standardized electrode width η(=Lt/Pt) defined bythe electrode finger width Lt of the IDT electrode and the electrodefinger pitch Pt to 0.5≦η≦0.7, thereby realizing a frequency deviation ofmaximum 830 ppm at normal temperature (25° C.).

A SAW resonator is also known in which grooves are formed in the surfaceof a quartz crystal substrate between electrode fingers constituting anIDT and between conductor strips constituting a reflector (for example,see JP-B-2-7207 (JP-A-57-5418) and Manufacturing Conditions andCharacteristics of Groove-type SAW Resonator (IECE, Technical ResearchReport MW82-59 (1982)). JP-B-2-7207 (JP-A-57-5418) describes a SAWresonator in which an IDT and a reflector are formed of aluminumelectrodes on an ST cut X-propagation quartz crystal substrate, andgrooves are formed in the corresponding regions of the quartz crystalsubstrate between electrode fingers constituting the IDT and betweenconductor strips (electrode fingers) constituting the reflector, therebyrealizing a high Q value, a low capacitance ratio, and low resonanceresistance. JP-B-2-7207 (JP-A-57-5418) describes a structure in whichthe groove of the IDT and the groove of the reflector have the samedepth and a structure in which the groove of the reflector is greater indepth than the groove of the IDT.

Manufacturing Conditions and Characteristics of Groove-type SAWResonator (IECE, Technical Research Report MW82-59 (1982) describes thecharacteristic of a groove-type SAW resonator using an ST cut quartzcrystal substrate. It has been reported that the frequency-temperaturecharacteristic changes depending on the depth of the grooves formed in aquartz crystal surface uncovered with the electrodes of the SAWpropagation substrate, and as the depth of the grooves increases, thepeak temperature Tp of an upward convex quadratic curve decreases.

A method which forms grooves in a piezoelectric substrate, such asquartz crystal, to adjust an effective thickness and to adjust afrequency is well known to those skilled in the art (for example, seeJP-A-2-189011, JP-A-5-90865, JP-A-1-231412, and JP-A-61-92011). In a SAWdevice described in JP-A-2-189011, the surface of the piezoelectricsubstrate having an IDT formed thereon is etched under the conditionthat the etching rate of the piezoelectric substrate is greater than theetching rate of the IDT, and fine adjustment is performed to lower thefrequency. In JP-A-5-90865, JP-A-1-231412, and JP-A-61-92011, similarly,the surface of a piezoelectric substrate is dry-etched with the IDTformed thereon as a mask, such that the frequency of the SAW device isshifted to a low-frequency band.

In a transversal SAW filter, a technique is known in which the surfaceof a piezoelectric substrate between electrode fingers of an IDTelectrode is etched to form grooves, thereby reducing an apparentpropagation speed (for example, see JP-A-10-270974). Thus, it ispossible to make the electrode finger pitch of the IDT electrode smallwithout changing the preliminary design of the SAW filter, therebyrealizing reduction in size of a chip.

In a SAW resonator which excites a shear wave called an SSBW (SurfaceSkimming Bulk Wave), it is known that an IDT electrode having astandardized electrode thickness (H/λ) of 2.0≦H/λ≦4.0% is formed ofaluminum on a rotation Y cut quartz crystal substrate in which a cutangle is −43° to −52° and a shear wave propagation direction is aZ′-axis direction (Euler angles (φ, θ, ψ)=(0°, 38≦θ≦47, 90°)), therebyrealizing a frequency-temperature characteristic of a cubic curve (forexample, see JP-B-1-34411). A shear wave (SH wave) propagates directlybelow the surface of the piezoelectric substrate in a state wherevibration energy is confined directly below the electrode. Thus, thereflection efficiency of the SAW by the reflector is unsatisfactorycompared to an ST cut quartz crystal SAW device in which a SAWpropagates along the substrate surface, making it difficult to realizereduction in size and a high Q value.

In order to solve this problem, a SAW device is suggested in which anIDT and a grating reflector are formed in the surface of a rotation Ycut quartz crystal substrate having Euler angles (φ, θ, ψ)=(0°,−64°<θ<−49.3°, 85°≦ψ≦95°) to excite an SH wave (for example, seeInternational Publication No. WO2005/099089A1). The SAW device sets theelectrode thickness H/λ standardized with the SAW wavelength λ to0.04<H/λ<0.12, thereby realizing reduction in size, a high Q value, andexcellent frequency stability.

In such a SAW device, in order to solve a problem in that the Q value orfrequency stability is deteriorated due to stress migration caused by alarge electrode thickness, a technique is suggested in which grooves areformed in the quartz crystal substrate between the electrode fingers ofthe IDT (for example, see JPA-2006-203408). When the depth of thegrooves is Hp and the thickness of a metal film of the IDT is Hm, theelectrode thickness H/λ standardized with the SAW wavelength λ is set to0.04<H/λ<0.12 (where H=Hp+Hm), such that the apparent thickness of themetal film can be made small. Thus, it is possible to suppress afrequency fluctuation due to stress migration at the time of electricalconduction, thereby realizing a SAW device having a high Q value andexcellent frequency stability.

During the mass production of SAW devices, when electrode fingers of anIDT are formed in the surface of a quartz crystal substrate by etching,if the thickness of the electrode fingers is large, a variation islikely to occur in the line occupancy (line space ratio) η of the IDTdue to side etching. As a result, if a variation occurs in the frequencyfluctuation with a change in the temperature of the SAW device, productreliability and quality are damaged. In order to solve this problem, aSAW device is known in which an in-plane rotation ST cut quartz crystalsubstrate having Euler angles (φ, θ, ψ)=(0°, 95°≦θ≦155°, 33°≦|ψ|≦46°) isused, a SAW stopband upper limit mode is excited, andinter-electrode-finger grooves are formed in the surface of the quartzcrystal substrate between electrode fingers of an IDT (for example, seeJP-A-2009-225420).

When the frequency-temperature characteristic of the SAW device is aquadratic curve in the operation temperature range, it is difficult torealize minimization of a frequency fluctuation range or an inflectionpoint. Accordingly, a SAW device is suggested in which, in order toobtain a frequency-temperature characteristic of a cubic curve, an IDTelectrode is formed on an LST cut quartz crystal substrate through avoid layer and a dielectric film to excite a leaky SAW (for example, seeJapanese Patent No. 3851336). Japanese Patent No. 3851336 describesthat, in a SAW device using a Rayleigh wave, a quartz crystal substratehaving a cut angle such that a frequency-temperature characteristicexpressed by a cubit curve is realized could not be found.

In an ST cut quartz crystal SAW resonator or the like, in order toincrease the Q value without deteriorating an excellentfrequency-temperature characteristic, an inclined IDT is known in whichan IDT and a reflector are arranged on the surface of a quartz crystalsubstrate to be inclined at a power flow angle PFA±3° with respect to aSAW phase velocity direction (for example, see Japanese Patent No.3216137 and JPA-2005-204275). In the SAW device having the inclined IDT,the IDT and the reflector are arranged so as to cover a SAW phasedirection and a vibration energy direction, such that the SAW can beefficiently reflected by the reflector. Thus, it is possible toefficiently perform energy confinement and to further increase the Qvalue.

PROBLEMS TO BE SOLVED BY THE INVENTION

As described above, many elements are related to thefrequency-temperature characteristic of the SAW device, and variousstudies are conducted for improvement. In particular, in a SAW using aRayleigh wave, it is considered that an increase in the thickness of theelectrode fingers constituting the IDT contributes to the improvement ofthe frequency-temperature characteristic. If the electrode thickness ofthe IDT simply increases, there is a problem in that deterioration infrequency stability or the like occurs due to stress migration at thetime of electrical conduction or side etching at the time of IDTformation. As a countermeasure, grooves are formed between the electrodefingers of the IDT in the surface of the quartz crystal substrate, andit is effective to suppress a frequency fluctuation by increasing theeffective thickness while making the electrode thickness small.

However, in all the SAW devices, excluding the SAW device described inJP-B-1-34411 which excites a leaky SAW, the frequency-temperaturecharacteristic in the operation temperature range is expressed by aquadratic curve, it is not difficult to sufficiently reduce a frequencyfluctuation range or to realize an inflection point. For this reason, itmay be impossible to sufficiently cope with recent requirements for aSAW device, such as expansion or high-precision performance of anoperation temperature range, long-term operation stability in anenvironment in which temperature undergoes severe changes, and the like.

SUMMARY OF THE INVENTION

The invention is made by considering the above-described problems, andthe object thereof is to provide a SAW device, such as a resonator or anoscillator, capable of exhibiting an excellent frequency-temperaturecharacteristic with a very small frequency fluctuation in an operationtemperature range, having an excellent environment-resistantcharacteristic ensuring a stable operation even in an environment inwhich a temperature varies extremely, and realizing a high Q value.

With regard to a SAW resonator in which an in-plane rotation ST cutquartz crystal substrate is used, an IDT which excites a SAW in astopband upper end mode is formed on the surface of the quartz crystalsubstrate, and the surface of the quartz crystal substrate betweenelectrode fingers constituting the IDT is recessed to form grooves, theinventors have verified the relationship between parameters, such as thewavelength λ of the SAW, the depth G of the grooves, the electrodethickness H of the IDT, and the line occupancy η of the electrodefingers, and the frequency-temperature characteristic. As a result, theinventors have devised a novel SAW resonator which can realizeminimization of a frequency fluctuation range and an inflection point inthe operation temperature range.

A SAW resonator according to a new embodiment (hereinafter, referred toas a SAW resonator of this embodiment) includes an IDT which is providedon a quartz crystal substrate having Euler angles (−1.5°≦φ≦1.5°,117°≦θ≦142°, 42.79°≦|ψ|≦49.57°), and excites a SAW in a stopband upperend mode. The quartz crystal substrate between electrode fingersconstituting the IDT is depressed to form inter-electrode-fingergrooves.

When the wavelength of the SAW is λ, and the depth of theinter-electrode-finger groove is G, the relationship 0.01λ≦G issatisfied.

When the line occupancy of the IDT is η, the depth G of theinter-electrode-finger grooves and the line occupancy η satisfy thefollowing relationships.Equation 1−2.0000×G/λ+0.7200≦η≦−2.5000×G/λ+0.7775 where 0.0100λ≦G≦0.0500λ  (1),Equation 2−3.5898×G/λ+0.7995≦η≦−2.5000×G/λ+0.7775 where 0.0500λ≦G≦0.06952  (2)

In the SAW resonator of this embodiment, the depth G of theinter-electrode-finger grooves may satisfy the relationship0.01λ≦G≦0.0695λ. If the depth G of the inter-electrode-finger grooves isset within this range, it is possible to suppress a frequencyfluctuation in an operation temperature range (for example, −40° C. to+85° C.) to be very small, and even when a manufacturing variationoccurs in the depth of the inter-electrode-finger grooves, it ispossible to suppress the shift amount of a resonance frequency betweenindividual SAW resonators within a correctable range.

In the SAW resonator of this embodiment, when the electrode thickness ofthe IDT is H, the relationship 0<H≦0.035λ may be satisfied. Therefore, asatisfactory frequency-temperature characteristic in an operationtemperature range is realized, and deterioration in anenvironment-resistant characteristic which may occur when the electrodethickness is large is prevented.

In the SAW resonator of this embodiment, the line occupancy η maysatisfy the following relationship.Equation 3η=−1963.05×(G/λ)³+196.28×(G/λ)²−6.53×(G/λ)−135.99×(H/λ)²+5.817×(H/λ)+0.732−99.99×(G/λ)×(H/λ)  (3)

Therefore, it is possible to suppress a secondary temperaturecoefficient of the frequency-temperature characteristic to be small.

In the SAW resonator of this embodiment, the sum of the depth G of theinter-electrode-finger grooves and the electrode thickness H may satisfythe relationship 0.0407λ≦G+H. Therefore, a high Q value is obtainedcompared to the related art which uses resonance in a stopband lower endmode with no grooves between electrode fingers.

FIG. 1 shows an example of a SAW resonator of this embodiment. As shownin FIG. 1(A), a SAW resonator 1 of this embodiment has a rectangularquartz crystal substrate 2, and an IDT 3 and a pair of reflectors 4 and4 which are formed on the principal surface of the quartz crystalsubstrate.

For the quartz crystal substrate 2, an in-plane rotation ST cut quartzcrystal substrate which is expressed by Euler angles (−1.5°≦φ≦1.5°,117°≦θ≦142°, 42.79°≦|ψ|≦49.57°) is used. Here, the Euler angles will bedescribed. A substrate which is expressed by Euler angles (0°, 0°, 0°)becomes a Z cut substrate which has a principal surface perpendicular tothe Z axis. Of the Euler angles (φ, θ, ψ), φ relates to the firstrotation of the Z cut substrate, and is a first rotation angle with theZ axis as a rotation axis. The rotation direction from the +X axis tothe +Y axis is defined as a positive rotation angle. Of the Eulerangles, θ relates to the second rotation after the first rotation of theZ cut substrate, and is a second rotation angle with the X axis afterthe first rotation as a rotation axis. The rotation direction from the+Y axis after the first rotation to the +Z axis is defined as a positiverotation angle. The cut plane of the piezoelectric substrate isdetermined by the first rotation angle φ and the second rotation angleθ. Of the Euler angles, ψ relates to the third rotation after the secondrotation of the Z cut substrate, and is a third rotation angle with theZ axis after the second rotation as a rotation axis. The rotationdirection from the +X axis after the second rotation to the +Y axisafter the second rotation is defined as a positive rotation angle. TheSAW propagation direction is expressed by the third rotation angle ψwith respect to the X axis after the second rotation.

As shown in FIG. 2, when three crystal axes perpendicular to quartzcrystal, that is, an electrical axis, a mechanical axis, and an opticalaxis are respectively expressed by the X axis, the Y axis, and the Zaxis, the in-plane rotation ST cut quartz crystal substrate is cut froma wafer 5 which has an XZ′ plane perpendicular to the Y′ axis of thecoordinate axes (X,Y′,Z′) obtained by rotating an XZ plane 5 aperpendicular to the Y axis at an angle θ′ (°) from the +Z axis to the−Y axis with the X axis as a rotation axis. The quartz crystal substrate2 is cut and individualized from the wafer 5 along new coordinate axes(X′,Y′,Z″) at an angle +ψ (or −ψ) (°) from the +X axis to the +Z′ axiswith the Y′ axis as a rotation axis. The direction from the +X axis tothe +Z′ axis is defined as positive. At this time, the long side (orshort side) of the quartz crystal substrate 2 may be arranged alongeither the X′-axis direction or the Z″-axis direction. The angle θ′ andθ of the Euler angles satisfy the relationship θ′=θ−90°.

An IDT 3 has a pair of interdigital transducers 3 a and 3 b whichrespectively have a plurality of electrode fingers 6 a and 6 b, and busbars 7 a and 7 b connecting the base portions of the electrode fingerstogether. The electrode fingers 6 a and 6 b are arranged such that theextension direction thereof is perpendicular to the propagationdirection X′ of the SAW which is excited by the IDT. The electrodefingers 6 a of the interdigital transducer 3 a and the electrode fingers6 b of the interdigital transducer 3 b are arranged with a given pitchalternately and at a predetermined interval. As shown in FIG. 1(B),inter-electrode-finger grooves 8 having a given depth are recessed inthe surface of the quartz crystal substrate 2 which is exposed betweenthe electrode fingers 6 a and 6 b by removing the surface throughetching or the like.

A pair of reflectors 4 and 4 are arranged outside the IDT 3 with the IDTsandwiched therebetween along the SAW propagation direction X′. Thereflectors 4 respectively have a plurality of conductor strips 4 a and 4a arranged with a given pitch in the SAW propagation direction X′.Similarly to the electrode fingers of the IDT 3, the conductor stripsare arranged such that the extension direction thereof is perpendicularto the SAW propagation direction X′. As shown in FIG. 1(B),inter-conductor-strip grooves 9 having a given depth are recessed in thesurface of the quartz crystal substrate 2 which is exposed between theconductor strips 4 a and 4 a by removing the surface through etching orthe like.

In this embodiment, the electrode fingers 6 a and 6 b and the conductorstrips 4 a and 4 a are formed of a metal film using, for example, Al oran alloy mainly containing Al to have the same thickness H, and may becollectively referred to as electrode fingers. Theinter-electrode-finger grooves 8 and the inter-conductor-strip grooves 9are formed to have the same depth G. Grooves are recessed between theoutermost electrode fingers 6 a (or 6 b) of the IDT 3 and the conductorstrips 4 a and 4 a of the reflectors 4 and 4 adjacent to the electrodefingers by removing the surface of the quartz crystal substrate to havethe same depth as the inter-conductor-strip grooves.

The SAW resonator 1 configured as above excites a Rayleigh-type SAWwhich has vibration displacement components in both the X′-axisdirection and the Y′-axis direction of the quartz crystal substrate 2.In the quartz crystal substrate 2 having the above-described Eulerangles, the SAW propagation direction is shifted from the X axis servingas the crystal axis of quartz crystal, making it possible to excite theSAW in the stopband upper end mode.

The Euler angles (φ, θ, ψ) of the quartz crystal substrate 2 wereselected as follow. In general, the frequency-temperature characteristicof the SAW resonator is expressed by the following expression.Δf=α×(T−T0)+β×(T−T0)²

Here, Δf is a frequency change amount (ppm) between a temperature T anda peak temperature T0, α is a primary temperature coefficient (ppm/°C.), β is a secondary temperature coefficient (ppm/° C.²), T is atemperature, and T0 is a temperature (peak temperature) at which afrequency is maximum. The absolute value of the secondary temperaturecoefficient β is set to be minimum, preferably, equal to or smaller than0.01 (ppm/° C.²), and more preferably, substantially zero, such that afrequency-temperature characteristic shows a cubic curve, a frequencyfluctuation becomes small even in a wide operation temperature range,thereby obtaining high frequency stability.

First, the Euler angles of the quartz crystal substrate 2 were set to(0°,123°,ψ), and the relationship between the Euler angle ψ and thedepth G of the inter-electrode-finger grooves when the line occupancy ηresulting in β=±0.01 (ppm/° C.²) has been obtained was simulated. TheEuler angle ψ was appropriately selected such that the absolute value ofthe secondary temperature coefficient β became 0.01 (ppm/° C.²). As aresult, the range of the Euler angle ψ for obtaining the secondarytemperature coefficient β of −0.01≦β≦0.01 under the above-describedcondition could be determined to 43°<ψ<45°.

As shown in FIG. 1(C), the line occupancy η of the IDT 3 is a valueobtained by dividing an electrode finger width L by an electrode fingerpitch λ/2(=L+S). FIG. 1(D) illustrates a method of specifying the lineoccupancy η of the IDT 3 in a trapezoidal cross-section which will beformed when the electrode fingers 6 a and 6 b of the IDT 3 and theinter-electrode-finger grooves 8 are manufactured by a photolithographytechnique and an etching technique. In this case, the line occupancy ηis calculated on the basis of the electrode finger width L and aninter-electrode-finger groove width S measured at a height half the sum(G+H) of the depth G of the inter-electrode-finger grooves from thebottom of the inter-electrode-finger grooves 8 and the electrodethickness H.

Next, when the cut angle and the quartz crystal substrate 2 and the SAWpropagation direction were (0,θ,ψ) in the Euler angle expression, thedepth G of the inter-electrode-finger grooves was 0.04λ, the thickness Hof the electrode fingers was 0.02λ, and the line occupancy η was 0.6383by Expression (3), changes in the secondary temperature coefficient βdepending on the Euler angle θ were simulated. The Euler angle ψ wasappropriately selected in the above-described range 43°<ψ<45° such thatthe absolute value of the secondary temperature coefficient β wasminimum on the basis of the set angle of the angle θ. As a result, ifthe Euler angle θ was within the range of 117°≦θ≦142°, it was confirmedthat, even when the thickness H of the electrode fingers, the depth G ofthe inter-electrode-finger grooves, and the line occupancy η werechanged, the absolute value of the secondary temperature coefficient βwas within the range of 0.01 (ppm/° C.²).

Next, the quartz crystal substrate 2 was) (φ,123°,43.77°) in the Eulerangle expression, the depth G of the inter-electrode-finger grooves was0.04λ, the thickness H of the electrode fingers was 0.02λ, and the lineoccupancy η was 0.65, changes of the secondary temperature coefficient βdepending on the Euler angle φ were simulated. As a result, if the Eulerangle φ was within the range of −1.5°≦φ≦+1.5°, it was confirmed that theabsolute value of the secondary temperature coefficient β was within therange of 0.01 (ppm/° C.²).

A highly desirable relationship between the Euler angles θ and ψ suchthat a frequency fluctuation was minimum in an operation temperaturerange (−40° C. to +85° C.) was obtained by a simulation. In this case,the depth G of the inter-electrode-finger grooves and the thickness H ofthe electrode fingers were respectively G=0.04λ and H=0.02λ. As aresult, the Euler angle ψ increased within the above-described range ofthe Euler angle θ such that a cubic curve was drawn with an increase inthe Euler angle ψ. This relationship can be approximated by thefollowing expression.ψ=1.19024×10⁻³×θ³−4.48775×10⁻¹×θ²+5.64362×10¹×θ−2.32327×10³±1.0  Equation4

Thus, the Euler angle ψ becomes ψ=42.79° with respect to the lower limitvalue θ=117° of the Euler angle θ, and ψ=49.57° with respect to theupper limit value θ=142° of the Euler angle θ. Therefore, the Eulerangle ψ can be set to 42.79≦ψ≦49.57° within the range of 117°≦θ≦142°.

If the Euler angles of the quartz crystal substrate 2 are set in theabove-described manner, the SAW resonator 1 of this embodiment canrealize an excellent frequency-temperature characteristic in which theabsolute value of the secondary temperature coefficient β is equal to orsmaller than 0.01 (ppm/° C.²).

With regard to the SAW resonator 1 of this embodiment, afrequency-temperature characteristic was simulated under the followingconditions.

Basic Data of Saw Resonator 1 of this Embodiment

H: 0.02λ

G: change

IDT line occupancy η: 0.6

Reflector line occupancy ηr: 0.8

Euler angles: (0°,123°,43.5°)

Number of pairs of IDT: 120

Electrode finger cross width: 40λ (λ=10 μm)

Number of reflectors (per side): 60

Inclination angle of electrode fingers: none

The simulation result is shown in FIG. 3. As will be understood fromFIG. 3, the frequency-temperature characteristic substantially shows acubic curve in the operation temperature range (−40 to +85° C.), and thefrequency fluctuation range can be suppressed with a very smallfluctuation within 20 ppm.

With regard to the SAW resonator 1 showing the frequency-temperaturecharacteristic of FIG. 3, if the frequency, the equivalent circuitconstant, and the static characteristic are put together, Table 1 isobtained.

TABLE 1 F (MHz) Q γ CI (Ω) M AVG 318.25 13285 2476 21.8 5.4 Here, F is afrequency, Q is a Q value, γ is a capacitance ratio, CI is a CI (CrystalImpedance) value, and M is a figure of merit.

The SAW resonator 1 is preferably set such that the frequency ft2 of thestopband upper end of the IDT 3, the frequency fr1 of the stopband lowerend of the reflector 4, and the frequency fr2 of the stopband upper endof the reflector 4 satisfy the relationship fr1<ft2<fr2. FIG. 4 showsthe SAW reflection characteristics of the IDT 3 and the reflector 4depending on the frequency. As shown in FIG. 4, if the frequency ft2 isset between the frequency fr1 and the frequency fr2, the reflectioncoefficient of the reflector 4 becomes larger than the reflectioncoefficient of the IDT 3 at the frequency ft2. As a result, the SAW inthe stopband upper end mode excited from the IDT 3 is reflected from thereflector 4 to the IDT 3 with a higher reflection coefficient.Therefore, the vibration energy of the SAW can be efficiently confined,thereby realizing a low-loss SAW resonator 1.

The relationship between the Q value of the SAW resonator 1 and themagnitude (G+H) of a step formed by the height, that is, thickness H ofthe electrode fingers 6 a and 6 b and the depth G of theinter-electrode-finger grooves 8 was verified by a simulation. Forcomparison, with regard to a SAW resonator of the related art in whichno grooves are formed between the electrode fingers and resonance in thestopband upper end mode is used, the relationship between the Q valueand the height, that is, thickness of the electrode fingers wassimulated under the following conditions.

Basic Data of SAW Resonator of the Related Art

H: change

G: zero (none)

IDT line occupancy η: 0.4

Reflector line occupancy ηr: 0.3

Euler angles (0°,123°,43.5°)

Number of pairs of IDT: 120

Electrode finger cross width: 40λ (λ=10 μm)

Number of reflectors (per side): 60

Inclination angle of electrode fingers: none

The simulation result is shown in FIG. 5. In FIG. 5, a bold lineindicates the SAW resonator 1 of this embodiment, and a thin lineindicates the SAW resonator of the related art. As will be understoodfrom FIG. 5, in the SAW resonator 1 of this embodiment, a high Q valuecan be obtained in a region where the step (G+H) is equal to or greaterthan 0.0407λ (4.07%λ), compared to the SAW resonator of the related art.

On the other hand, in the SAW resonator of this embodiment, it wasascertained that a variation in the frequency-temperature characteristicoccurred between individuals. As described above, in this embodiment, anexcellent frequency-temperature characteristic of a cubic curve isrealized with the SAW wavelength λ, the depth G of theinter-electrode-finger grooves, and the IDT line occupancy η, and theelectrode finger thickness H as parameters. Thus, the inventors haveconsidered that manufacturing errors in the parameters have no influenceon a variation in the frequency-temperature characteristic, and haveverified the relationship.

At the time of the mass production of SAW devices, in general, theelectrode fingers of the IDT are formed by photoetching an electrodefilm, but it is postulated that the line width L has normally amanufacturing error of about 0.5%. In this case, it is considered thatthe IDT line occupancy η has a manufacturing variation at the samelevel. On the basis of the postulation, in the SAW resonator 1 of FIG.1, when the electrode finger thickness is H=2%λ, the groove depth isG=3.5%λ, the IDT line occupancy is η=0.63 (=63%), and when theparameters are shifted by ±0.005 (=±0.5%), the frequency-temperaturecharacteristics were calculated by simulations. FIG. 6 shows theresults.

In both cases, the frequency-temperature characteristic is expressed bya cubic curve having a maximum value, a minimum value, and an inflectionpoint between the maximum value and the minimum value in a usetemperature range. When η=0.63 (solid line), an exceptionalfrequency-temperature characteristic was shown in which the frequencyfluctuation in the use temperature range (−40° C. to +85° C.) was within±5 ppm, and the position of the inflection point, that is, theinflection-point temperature was substantially the center of the usetemperature range and substantially symmetric. Meanwhile, when η=0.625(fine broken line) and 0.635 (large broken line), it is understood thatthe frequency-temperature characteristic is deteriorated in which afrequency fluctuation increases to be equal to or greater than ±5 ppm inthe use temperature range, and the inflection-point temperature issignificantly shifted to the low-temperature side or thehigh-temperature side and asymmetric.

Next, the inventors have verified an influence of a shift in aninflection-point temperature on a frequency fluctuation in afrequency-temperature characteristic. FIG. 7 shows the relationshipbetween a change amount in the inflection-point temperature and adeviation in the frequency fluctuation when η=0.63 of FIG. 6. From FIG.7, it is understood that, if a variation occurs in the inflection-pointtemperature, a deviation in the frequency fluctuation increases, causingdeterioration in the frequency-temperature characteristic.

The inventors have verified an influence of a change in the depth G ofthe inter-electrode-finger grooves relating to the electrode fingerthickness H on a change amount in the inflection-point temperature dueto a shift (±0.005) in the IDT line occupancy η, that is, thefrequency-temperature characteristic. In this specification, it isassumed that a change amount in the inflection-point temperature due toa shift in the IDT line occupancy η is called inflection-pointsensitivity as an index representing the degree of deterioration in thefrequency-temperature characteristic.

First, in the SAW resonator 1 of FIG. 1, when the Euler angles of thequartz crystal substrate 2 are set to (0°, 123°, ψ), the electrodefinger thickness H is fixed to 1%λ, and the inter-electrode-fingergroove depth G changes by 1%λ in a range of 2%λ to 7%λ, the relationshipbetween the IDT line occupancy η such that the secondary temperaturecoefficient β of the frequency-temperature characteristic is equal to orsmaller than 0.01 and the inflection-point sensitivity due to a shift of±0.005 in the η value was calculated by a simulation. The result isshown in FIG. 8. As will be understood from FIG. 8, in both cases, as ηincreases, the inflection-point sensitivity decreases, such that thefrequency-temperature characteristic is improved.

Next, when the electrode finger thickness H is fixed to 1.5%λ, and thegroove depth G changes by 1%λ in a range of 2%λ to 7%λ, the relationshipbetween the IDT line occupancy η such that the secondary temperaturecoefficient β of the frequency-temperature characteristic is equal to orsmaller than 0.01 and the inflection-point sensitivity due to a shift of±0.005 in the η value was calculated by a simulation under the sameconditions as in FIG. 8. The result is shown in FIG. 9. From FIG. 9, itis understood that, in both cases, as η increases, the inflection-pointsensitivity decreases, such that the frequency-temperaturecharacteristic is improved.

FIG. 10 is a plot diagram of a change in the inflection-pointtemperature relating to the IDT line occupancy η in the simulationresult of FIG. 8. FIG. 10 shows that, as η increases, the change rate ofthe inflection-point temperature increases.

Thus, the inventors have verified the relationship between the electrodethickness H of the IDT 3 such that the inflection-point sensitivity dueto a shift in the IDT line occupancy η is improved, theinter-electrode-finger groove depth G, and the IDT line occupancy η inthe SAW resonator 1 of this embodiment. As a result, the inventors havefound that there is a specific relationship between theinter-electrode-finger groove depth G and the IDT line occupancy η so asto suppress a variation and deterioration in the frequency-temperaturecharacteristic. The inventors have devised the invention on the basis ofthe findings.

A SAW device according to an aspect of the invention includes apiezoelectric substrate having Euler angles (−1.5°≦φ≦1.5°, 117°≦θ≦142°,|ψ|≠90°×n (where n=0, 1, 2, 3)), and an IDT which has a plurality ofelectrode fingers on the principal surface of the piezoelectricsubstrate and excites a Rayleigh wave in a stopband upper end mode.Inter-electrode-finger grooves are recessed in the surface of thepiezoelectric substrate between adjacent electrode fingers of the IDT.The wavelength λ of the Rayleigh wave and the depth G of theinter-electrode-finger grooves satisfy the relationship 0.01λ≦G≦0.07λ.The line occupancy η of the IDT and the depth G of theinter-electrode-finger grooves satisfy the following relationships.−2.0000×G/λ+0.7200≦η≦−2.5000×G/λ+0.7775 where0.0100λ≦G≦0.0500λ  Equation 5−3.5898×G/λ+0.7995≦η≦−2.5000×G/λ+0.7775 where0.0500λ≦G≦0.0695λ  Equation 6

The thickness H of the electrode fingers and the line occupancy η of theIDT satisfy any one of the following relationships.

(1) 0.010λ≦H≦0.015λ and 0.575≦η≦0.741

(2) 0.015λ≦H≦10.020λ and 0.605≦η≦0.744

(3) 0.020λ≦H≦0.025λ and 0.605≦η≦0.745

(4) 0.025λ≦H≦0.030λ and 0.575≦η≦0.745

The relationship between the inter-electrode-finger groove depth G, theelectrode finger thickness H, and the IDT line occupancy η is defined inthe above-described relationship, such that the SAW device has asatisfactory frequency-temperature characteristic which is expressed bya curve constantly having a maximum value, a minimum value, and aninflection point between the maximum value and the minimum value in anoperation temperature range. Therefore, it is possible to suppress avariation in the inflection-point temperature between individuals due toa shift in the IDT line occupancy η caused by a manufacturing variation,thereby suppressing a variation and deterioration in thefrequency-temperature characteristic due to the variation in theinflection-point temperature.

The Euler angle ψ of the piezoelectric substrate may be within a rangeof 42.79°≦|ψ|≦149.57°. Therefore, an excellent frequency-temperaturecharacteristic having a very small frequency fluctuation is obtainedwithin a wide operation temperature range.

The Euler angle ψ of the piezoelectric substrate may be |ψ|=45°.Therefore, it is possible to further reduce a variation in theinflection-point temperature.

In an orthogonal coordinate in which the vertical axis is the lineoccupancy η (%) and the horizontal axis is the depth G (%λ) of theinter-electrode-finger grooves, the line occupancy η of the IDT may bein the following ranges.

(1) a range surrounded by a polygonal which connects points (0.74,1),(0.75,2), (0.74,3), (0.735,4), (0.741,5), (0.73,6), (0.73,7), (0.575,7),(0.615,6), (0.64,5), (0.66,4), (0.675,3), (0.703,2), and (0.725,1) whenthe electrode finger thickness H is 1.0 to 1.5% k

(2) a range surrounded by a polygonal which connects points (0.702,1),(0.718,2), (0.744,3), (0.731,4), (0.725,5), (0.73,6), (0.745,7),(0.605,7), (0.605,6), (0.63,5), (0.655,4), (0.675,3), (0.7,2), and(0.687,1) when the electrode finger thickness H is 1.5 to 2.0%λ

(3) a range surrounded by a polygonal which connects points (0.718,2),(0.744,3), (0.731,4), (0.725,5), (0.73,6), (0.745,7), (0.605,7),(0.605,6), (0.62,5), (0.645,4), (0.669,3), and (0.695,2) when theelectrode finger thickness H is 2.0 to 2.5%λ

(4) a range surrounded by a polygonal which connects points (0.745,1),(0.74,2), (0.74,3), (0.745,4), (0.725,5), (0.725,6), (0.725,7),(0.575,7), (0.575,6), (0.605,5), (0.635,4), (0.66,3), (0.695,2), and(0.715,1) when the electrode finger thickness H is 2.5 to 3.0%λ

The IDT line occupancy η and the electrode finger thickness H arespecified in the above-described manner, making it possible to improve avariation in the inflection-point temperature of thefrequency-temperature characteristic between individuals due to a shiftin the IDT line occupancy η caused by a manufacturing variation and tofurther suppress a variation and deterioration in thefrequency-temperature characteristic.

The IDT line occupancy η may satisfy the following relationship.η=−1963.05×(G/λ)³+196.28×(G/λ)²−6.53×(G/λ)−135.99×(H/λ)²+5.817×(H/λ)+0.732−99.99×(G/λ)×(H/λ)  Equation 7

Therefore, it is possible to suppress the secondary temperaturecoefficient of the frequency-temperature characteristic smaller, therebyobtaining an excellent frequency-temperature characteristic of a cubiccurve having a smaller frequency fluctuation.

The sum of the depth G of the inter-electrode-finger grooves and thethickness H of the electrode fingers may satisfy 0.0407λ≦G+H. Therefore,in the aspect of the invention which uses resonance in the stopbandupper end mode, a high Q value is obtained compared to a SAW resonatorof the related art which uses resonance in the stopband lower end modewith no grooves between the electrode fingers of the IDT.

The SAW device may further include a pair of reflectors whichrespectively have a plurality of conductor strips on the principalsurface of the quartz crystal substrate, and are arranged on both sidesof the IDT with the IDT sandwiched therebetween along the SAWpropagation direction. Inter-conductor-strip grooves may be recessed inthe surface of the quartz crystal substrate between adjacent conductorstrips of the reflectors. The angle between a first directionperpendicular to the electrode fingers and the conductor strips and anelectrical axis of the quartz crystal substrate may be the Euler angle ψof the quartz crystal substrate, at least a part of the IDT and thereflectors may be arranged in a second direction intersecting the firstdirection at an angle δ, and the angle δ may be set within a range of apower flow angle ±1° of the quartz crystal substrate. Therefore, it ispossible to improve the Q value.

The SAW device may further include an oscillation circuit which drivesthe IDT. Therefore, it is possible to obtain a SAW oscillator having avery small frequency fluctuation in a wide operation temperature range,a low CI value, and excellent oscillation stability.

According to another aspect of the invention, an electronic apparatusand a sensor apparatus include the above-described SAW device.Therefore, an electronic apparatus and a sensor apparatus which stablyexhibit satisfactory performance in a wide operation temperature rangewith high reliability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 (A) is a plan view showing the configuration of a SAW resonatorof this embodiment, (B) is a partial enlarged longitudinal sectionalview, (C) is a partial enlarged view, and (D) is a partial enlargedsectional view showing the shape of inter-electrode-finger grooves whichare formed by photolithography and etching techniques, and correspondingto (C).

FIG. 2 is an explanatory view schematically showing a quartz crystalsubstrate of this embodiment.

FIG. 3 is a diagram showing a frequency-temperature characteristic ofthis embodiment.

FIG. 4 is a diagram showing SAW reflection characteristics of an IDT anda reflector.

FIG. 5 is a diagram showing the relationship between a step betweenelectrode fingers of this embodiment and a Q value.

FIG. 6 is a diagram showing a variation in a frequency-temperaturecharacteristic due to a variation in an IDT line occupancy η of thisembodiment.

FIG. 7 is a diagram showing the relationship between a change amount inan inflection-point temperature and a deviation in a frequencyfluctuation in a frequency-temperature characteristic of thisembodiment.

FIG. 8 is a diagram showing the relationship between an IDT lineoccupancy η and inflection-point sensitivity in a frequency-temperaturecharacteristic when an electrode finger thickness is H=1%λ.

FIG. 9 is a diagram showing the relationship between an IDT lineoccupancy η and inflection-point sensitivity in a frequency-temperaturecharacteristic when an electrode finger thickness is H=1.5%λ.

FIG. 10 is a diagram showing the relationship between an IDT lineoccupancy η and an inflection-point temperature in afrequency-temperature characteristic when an electrode finger thicknessis H=1%λ.

FIGS. 11 (A) to (E) are diagrams showing the relationship between aninter-electrode-finger groove depth G and an IDT line occupancy η suchthat inflection-point sensitivity can be improved when an electrodefinger thickness is H=1%λ, 1.5%λ, 2%λ, 2.5%λ, and 3%λ.

FIGS. 12 (A) to (D) are diagrams showing the relationship between aninter-electrode-finger groove depth G and an IDT line occupancy η suchthat inflection-point sensitivity can be improved when an electrodefinger thickness is in a range of H=1 to 1.5%λ, 1.5 to 2%λ, 2 to 2.5%λ,and 2.5 to 3.0%λ.

FIG. 13 is a diagram showing the improvement of a variation in afrequency-temperature characteristic due to a variation in an IDT lineoccupancy η in a SAW resonator according to an embodiment of theinvention.

FIGS. 14 (A) and (B) are plan views showing SAW resonators havinginclined IDTs of different structures according to a second example ofthe invention.

FIG. 15 (A) is a plan view showing a SAW oscillator according to anembodiment, and (B) is a longitudinal sectional view taken along theline B-B.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, preferred examples of the invention will be described indetail with reference to the accompanying drawings. In the accompanyingdrawings, the same or similar constituent elements are represented bythe same or similar reference numerals.

A SAW resonator which is a first example of a SAW device according tothe invention has the same basic configuration as the SAW resonator 1shown in FIG. 1, and description thereof will be provided with referenceto FIG. 1. That is, the SAW resonator 1 of this example has arectangular quartz crystal substrate 2, and an IDT 3 and a pair ofreflectors 4 and 4 which are formed on the principal surface of thequartz crystal substrate. The quartz crystal substrate 2 uses a quartzcrystal substrate having Euler angles (−1.5°≦φ≦1.5°, 117°≦θ≦142°, ψ).The Euler angle ψ is set such that |ψ|≠90°×n (where n=0, 1, 2, 3) issatisfied.

The IDT 3 has a pair of interdigital transducers 3 a and 3 b whichrespectively have a plurality of electrode fingers 6 a and 6 b, and busbars 7 a and 7 b connecting the base portions of the electrode fingers 6a and 6 b. The electrode fingers 6 a and 6 b are arranged such that theextension direction thereof is perpendicular to the propagationdirection of a SAW which is excited by the IDT. The electrode fingers 6a of the interdigital transducer 3 a and the electrode fingers 6 b ofthe interdigital transducer 3 b are arranged with a given pitchalternately and at a predetermined interval. Inter-electrode-fingergrooves 8 having a given depth are recessed in the surface of the quartzcrystal substrate 2 exposed between the electrode fingers 6 a and 6 b byremoving the surface through etching or the like.

A pair of reflectors 4 and 4 are arranged outside the IDT 3 with the IDTsandwiched therebetween along the SAW propagation direction. Thereflectors 4 respectively have a plurality of conductor strips(electrode fingers) 4 a and 4 a arranged with a given pitch in the SAWpropagation direction. The conductor strips are arranged such that theextension direction thereof is perpendicular to the SAW propagationdirection. Inter-conductor-strip grooves 9 are recessed in the surfaceof the quartz crystal substrate 2 exposed between the conductor strips 4a and 4 a by removing the surface through etching or the like.

The electrode fingers and the conductor strips are formed of a metalfilm using, for example, Al or an alloy mainly containing Al to have thesame thickness H. The inter-electrode-finger grooves and theinter-conductor-strip grooves are formed to have the same depth G.Similarly, grooves having the same depth as the inter-electrode-fingergrooves are formed between the innermost conductor strips of thereflectors 4 and 4 and the outermost electrode fingers 6 a (or 6 b) ofthe IDT 3 at a predetermined interval by removing the surface of thequartz crystal substrate.

With this configuration, the SAW resonator 1 excites a Rayleigh-type SAWwhich has vibration displacement components in both the X′-axisdirection and the Y′-axis direction of the quartz crystal substrate 2.With the use of the quartz crystal substrate 2 having theabove-described Euler angles, the SAW propagation direction is shiftedfrom the X axis which is the crystal axis of quartz crystal, therebyexciting the SAW in the stopband upper end mode.

At the time of the mass production of SAW devices, if it is supposedthat a manufacturing error in the line width L of the electrode fingersis 0.5%, it is postulated that a manufacturing variation occurs in theIDT line occupancy η at the same level. In this example, it ispostulated that a manufacturing variation of ±0.005 occurs in the designvalue of the IDT line occupancy η. On the basis of the postulation, theSAW resonator 1 sets the inter-electrode-finger groove depth G, theelectrode finger thickness H, and the IDT line occupancy η such that thefrequency-temperature characteristic is expressed by a curve constantlyhaving a maximum value, a minimum value, and an inflection point betweenthe maximum value and the minimum value in the operation temperaturerange.

In the SAW resonator 1 of this example, the relationship between theinter-electrode-finger groove depth G and the IDT line occupancy η suchthat the inflection-point sensitivity was improved was studied. It isdetermined that the inflection-point sensitivity is improved when avariation in the inflection-point temperature due to a shift of ±0.005with respect to the design value of the IDT line occupancy η is small.When the Euler angles of the quartz crystal substrate 2 are set to (0°,123°, ψ), and the electrode finger thickness H is 1%λ, 1.5%λ, 2%λ,2.5%λ, and 3%λ, the relationship between the inter-electrode-fingergroove depth G and the IDT line occupancy η such that the secondarytemperature coefficient β of the frequency-temperature characteristic isequal to or smaller than 0.01 was calculated by a simulation.

The inter-electrode-finger groove depth G is set in a range of 1%λ to7%λ such that a frequency fluctuation can be suppressed to be small, forexample, within a wide operation temperature range of −40° C. to +85°C., and even when a manufacturing variation occurs in the groove depthG, a shift amount in the resonance frequency between individual SAWresonators can be suppressed in a correctable range. The electrodefinger thickness H is selected in a range of 1%λ to 3%λ such that asatisfactory frequency-temperature characteristic can be realized withinan operation temperature range and deterioration in anenvironment-resistant characteristic when the electrode finger thicknessincreases can be prevented.

The simulation result is shown in FIG. 11(A) to (E). FIG. 11A is a plotdiagram of the points of the inter-electrode-finger groove depth G (%λ)and the IDT line occupancy η such that the secondary temperaturecoefficient of the frequency-temperature characteristic is β=0.01 whenthe electrode finger thickness is H=1.0%λ in an orthogonal coordinate inwhich the vertical axis is the line occupancy λ (%) and the horizontalaxis is the inter-electrode-finger groove depth G (%λ). FIG. 11(B) to(E) are plot diagrams of the points of the inter-electrode-finger groovedepth G (%λ) and the IDT line occupancy η such that the secondarytemperature coefficient of the frequency-temperature characteristic isβ=0.01 when the electrode finger thickness is H=1.5%λ, 2.0%λ, 2.5%λ, and3.0%λ in the same orthogonal coordinate as in FIG. 11(A).

As will be understood from FIG. 11(A) to (E), in all cases, there is arange, in which the secondary temperature coefficient β of thefrequency-temperature characteristic is equal to 0.01 or less, betweenthe inter-electrode-finger groove depth G, the electrode fingerthickness H, and the IDT line occupancy η. In FIG. 11(A) to (E), theinflection-point sensitivity is improved within a range surrounded by apolygonal connecting the plotted points, such that, even when the IDTline occupancy η is shifted from the design value, a variation in thefrequency-temperature characteristic is suppressed.

The simulation results of FIG. 11(A) to (E) can be put together asTables 2 to 5.

TABLE 2 G η 1 2 3 4 5 6 7 Upper 0.725 0.703 0.675 0.66 0.64 0.615 0.575Limit Lower 0.74 0.75 0.74 0.735 0.741 0.73 0.73 Limit

TABLE 3 G η 1 2 3 4 5 6 7 Upper 0.687 0.7 0.675 0.655 0.63 0.605 0.605Limit Lower 0.702 0.718 0.744 0.731 0.725 0.73 0.745 Limit

TABLE 4 G η 1 2 3 4 5 6 7 Upper 0.715 0.695 0.669 0.645 0.62 0.605 0.605Limit Lower 0.702 0.718 0.744 0.731 0.725 0.73 0.745 Limit

TABLE 5 G η 1 2 3 4 5 6 7 Upper 0.715 0.695 0.66 0.635 0.605 0.575 0.575Limit Lower 0.745 0.74 0.74 0.745 0.725 0.725 0.725 Limit

Table 2 shows a range where the polygonal of FIG. 11(A) and thepolygonal of FIG. 11(B) overlap each other. Similarly, Tables 3 to 5show a range where the polygonal of FIG. 11(B) and the polygonal of FIG.11(C) overlap each other, a range where the polygonal of FIG. 11(C) andthe polygonal of FIG. 11(D) overlap each other, and a range where thepolygonal of FIG. 11(D) and the polygonal of FIG. 11(E) overlap eachother.

In this example, from Tables 2 to 5, the electrode finger thickness Hand the IDT line occupancy η satisfy any one of the followingrelationships.

(1) 0.010λ≦H≦0.015λ and 0.575≦η≦0.741

(2) 0.015λ≦H≦0.020λ and 0.605≦η≦0.744

(3) 0.020λ≦H≦0.025λ and 0.605≦η≦0.745

(4) 0.025λ≦H≦0.030λ and 0.575≦η≦0.745

Thus, it is possible to improve the inflection-point sensitivity withrespect to a shift in the IDT line occupancy η due to a manufacturingvariation. Therefore, a SAW device is obtained which has a satisfactoryfrequency-temperature characteristic expressed by a curve constantlyhaving a maximum value, a minimum value, and an inflection point betweenthe maximum value and the minimum value within an operation temperaturerange, and suppresses a variation and deterioration in thefrequency-temperature characteristic due to a shift in the IDT lineoccupancy η between individuals.

As in FIG. 11(A) to (E), FIG. 12(A) to (D) are plot diagrams of thepoints of Tables 2 to 5 in an orthogonal coordinate in which thevertical axis is the line occupancy η (%) and the horizontal axis is theinter-electrode-finger groove depth G (%λ). That is, FIG. 12(A) is aplot diagram of the points of the inter-electrode-finger groove depth G(%λ) and the IDT line occupancy η such that the secondary temperaturecoefficient of the frequency-temperature characteristic is β=0.01 in arange of the electrode finger thickness H=1.0 to 1.5%λ. Theinflection-point sensitivity is improved in a range surrounded by apolygonal connecting the points (0.74,1), (0.75,2), (0.74,3), (0.735,4),(0.741,5), (0.73,6), (0.73,7), (0.575,7), (0.615,6), (0.64,5), (0.66,4),(0.675,3), (0.703,2), and (0.725,1).

FIG. 12(B) is a plot diagram of the points of the inter-electrode-fingergroove depth G (% k) and the IDT line occupancy η such that thesecondary temperature coefficient of the frequency-temperaturecharacteristic is β=0.01 in a range of the electrode finger thicknessH=1.5 to 2.0%λ. The inflection-point sensitivity is improved in a rangesurrounded by a polygonal which connects the points (0.702,1),(0.718,2), (0.744,3), (0.731,4), (0.725,5), (0.73,6), (0.745,7),(0.605,7), (0.605,6), (0.63,5), (0.655,4), (0.675,3), (0.7,2), and(0.687,1).

FIG. 12(C) is a plot diagram of the points of the inter-electrode-fingergroove depth G (%λ) and the IDT line occupancy η such that the secondarytemperature coefficient of the frequency-temperature characteristic isβ=0.01 in a range of the electrode finger thickness H=2.0 to 2.5%λ. Theinflection-point sensitivity is improved in a range surrounded by apolygonal which connects the points (0.718,2), (0.744,3), (0.731,4),(0.725,5), (0.73,6), (0.745,7), (0.605,7), (0.605,6), (0.62,5),(0.645,4), (0.669,3), and (0.695,2).

FIG. 12(D) is a plot diagram of the points of the inter-electrode-fingergroove depth G (%λ) and the IDT line occupancy η such that the secondarytemperature coefficient of the frequency-temperature characteristic isβ=0.01 in a range of the electrode finger thickness H=2.5 to 3.0%λ. Theinflection-point sensitivity is improved in a range surrounded by apolygonal which connects the points (0.745,1), (0.74,2), (0.74,3),(0.745,4), (0.725,5), (0.725,6), (0.725,7), (0.575,7), (0.575,6),(0.605,5), (0.635,4), (0.66,3), (0.695,2), and (0.715,1).

If the inter-electrode-finger groove depth G and the electrode fingerthickness H, and the IDT line occupancy η are specified in the range ofthe polygonal shown in each of FIG. 12(A) to (D), the SAW resonator ofthis example exhibits a satisfactory frequency-temperaturecharacteristic of a cubic curve constantly having a maximum value, aminimum value, and the inflection point between the maximum value andthe minimum value within the operation temperature range, and cansuppress a variation and deterioration in the frequency-temperaturecharacteristic due to a shift in the IDT line occupancy η caused by amanufacturing variation.

With regard to the SAW resonator 1 of this example, thefrequency-temperature characteristic was evaluated in a wide operationtemperature range of −10° C. to +125° C. The Euler angles (φ, θ, ψ) ofthe quartz crystal substrate 2 were set to (0°, 123°, |45°|). Theelectrode finger thickness, the inter-electrode-finger groove depth, andthe IDT line occupancy were set to H=1%λ, G=3%λ, and η=0.71 (=71%) fromthe ranges defined in Table 2 and FIG. 12(A). Even when the IDT lineoccupancy is η=0.705 (=70.5%) and η=0.715 (=71.5%) by ±0.005 (=±0.5%)from the η value, the frequency-temperature characteristics weresimulated. The result is shown in FIG. 13.

In FIG. 13, the frequency-temperature characteristic (solid line) ofη=0.71 is expressed by a cubic curve having a maximum value, a minimumvalue, and an inflection point between the maximum value and the minimumvalue in the operation temperature range. An exceptionalfrequency-temperature characteristic is shown in which a frequencyfluctuation is within a range of about −7 ppm to +5 ppm, and theinflection-point temperature is substantially the center of operationtemperature range around 60° C. and substantially symmetric as a whole.

The frequency-temperature characteristics of η=0.705 (fine broken line)and 0.715 (large broken line) are expressed by a cubic curve having amaximum value, a minimum value, and an inflection point between themaximum value and the minimum value in the operation temperature range.In all cases, an exceptional frequency-temperature characteristic isshown in which a frequency fluctuation is within a range of about −7 ppmto +5 ppm, and the inflection-point temperature is substantially thecenter of the operation temperature range around 60° C. andsubstantially symmetric. As will be easily understood from FIG. 13,according to this example, even when a shift of about ±0.005 occurs inthe IDT line occupancy η, it could be confirmed that there is littlevariation or deterioration in the frequency-temperature characteristic.

With regard to the SAW resonator of this example, similarly to the SAWresonator of this embodiment, it is preferable that the line occupancy ηof the IDT 3 is set to satisfy the following relationship.η=−1963.05×(G/λ)³+196.28×(G/λ)²−6.53×(G/λ)−135.99×(H/λ)²+5.817×(H/λ)+0.732−99.99×(G/λ)×(H/λ)  Equation 8

Thus, it is possible to suppress the secondary temperature coefficientof the frequency-temperature characteristic to be smaller, and thus tofurther reduce a frequency fluctuation, thereby obtaining a moreexcellent frequency-temperature characteristic of a cubic curve.

In the SAW resonator of this example, similarly to the SAW resonator ofthis embodiment, it is preferable that the sum of the depth G of theinter-electrode-finger grooves 8 and the thickness H of the electrodefingers 6 a and 6 b is set to satisfy 0.0407λ≦G+H. Thus, in this examplewhich uses resonance in the stopband upper end mode, a high Q value isobtained compared to the SAW resonator of the related art which usesresonance in the stopband lower end mode with no grooves between theelectrode fingers of the IDT.

FIGS. 14(A) and (B) show a second example of a SAW resonator having aninclined IDT according to the invention. As in the first example, a SAWresonator 21 ₁ of FIG. 14(A) has an inclined IDT 23 ₁ and a pair ofreflectors 24 ₁ and 24 ₁ on the principal surface of a quartz crystalsubstrate 22 ₁ expressed by Euler angles (−1.5°≦φ≦1.5°, 117°≦θ142°,42.79°≦|ψ|≦49.57°). The quartz crystal substrate 22 ₁ is such that thelongitudinal direction thereof is aligned in a direction inclined by apower flow angle (PFA) δ° in the propagation direction of energy withrespect to the X′ axis which is the propagation direction of the phasevelocity of the SAW excited by the IDT 23 ₁.

The IDT 23 ₁ has a pair of interdigital transducers 23 a ₁ and 23 b ₁which respectively have a plurality of electrode fingers 25 a ₁ and 25 b₁, and bus bars 26 a ₁ and 26 b ₁ connecting the base portions of theelectrode fingers. A pair of reflectors 24 ₁ and 24 ₁ are arranged onboth sides of the IDT 23 ₁ with the IDT sandwiched therebetween alongthe SAW propagation direction, and respectively have a plurality ofconductor strips 24 a ₁ and 24 a ₁ arranged in the SAW propagationdirection. The electrode fingers 25 a ₁ and 25 b ₁ and the conductorstrips 24 a ₁ are arranged such that the extension direction thereof isperpendicular to the X′ axis inclined at the power flow angle (PFA) δ°.

As in the first example, inter-electrode-finger grooves are recessed inthe surface of the quartz crystal substrate 22 ₁ exposed between theelectrode fingers 25 a ₁ and 25 b ₁. Similarly, inter-conductor-stripgrooves are recessed in the surface of the quartz crystal substrate 22 ₁between the conductor strips 24 a ₁ and 24 a ₁.

If at least a part of the IDT and the reflectors are arranged in adirection intersecting the X′-axis direction at the power flow angle δ,the SAW device 21 ₁ exhibits the same functional effects as in the firstexample, thereby further increasing the Q value. Thus, a lower-loss SAWresonator is realized.

A SAW resonator 21 ₂ of FIG. 14(B) has an inclined IDT 23 ₂ and a pairof reflectors 24 ₂ and 24 ₂ having a different configuration from FIG.14(A) on the principal surface of the quartz crystal substrate 22 ₂. Thequartz crystal substrate 22 ₂ is such that the longitudinal directionthereof is aligned along the X′ axis which is the propagation directionof the phase velocity of the SAW excited by the IDT 23 ₂.

The IDT 23 ₂ has a pair of interdigital transducers 23 a ₂ and 23 b ₂which respectively have a plurality of electrode fingers 25 a ₂ and 25 b₂, and bus bars 26 a ₂ and 26 b ₂ connecting the base portions of theelectrode fingers. A pair of reflectors 24 ₂ and 24 ₂ are arranged onboth sides of the IDT 23 ₂ with the IDT sandwiched therebetween alongthe SAW propagation direction, and respectively have a plurality ofconductor strips 24 a ₂ and 24 a ₂ arranged in the SAW propagationdirection. The electrode fingers 25 a ₂ and 25 b ₂ and the conductorstrip 24 a ₂ are arranged such that the extension direction thereof isperpendicular to the X′ axis, and the bus bars 26 a ₂ and 26 b ₂ arealigned in a direction inclined at the power flow angle (PFA) δ° fromthe X′ axis.

As in the first example, inter-electrode-finger grooves are recessed inthe surface of the quartz crystal substrate 22 ₂ exposed between theelectrode fingers 25 a ₂ and 25 b ₂. Similarly, inter-conductor-stripgrooves are recessed in the surface of the quartz crystal substrate 22 ₂between the conductor strips 24 a ₂ and 24 a ₂.

In the SAW resonator 21 ₂ of this example, if at least a part of the IDTand the reflectors is arranged in a direction intersecting the X′-axisdirection at the power flow angle δ in the above-described manner, afunctional effect of realizing a satisfactory frequency-temperaturecharacteristic and a high Q value is exhibited, thereby furtherincreasing the Q value. Thus, a lower-loss SAW resonator is realized.

The invention may be applied to an oscillator in which the SAW resonatorof this embodiment and an oscillation circuit are incorporated. FIGS.15(A) and (B) show the configuration of an example of a SAW oscillatorwhich is a second example of a SAW device according to the invention. ASAW oscillator 31 of this example includes a SAW resonator 32 of thisembodiment, an IC (integrated circuit) 33 which serves as an oscillationcircuit to drive and control the SAW resonator, and a package 34 whichaccommodates the SAW resonator 32 and the IC 33. The SAW resonator 32and the IC 33 are surface-mounted on a bottom plate 34 a of the package34.

The SAW resonator 32 has the same configuration of the SAW resonator 11of the first example. The SAW resonator 32 has a quartz crystalsubstrate 35 which is expressed by the same Euler angles as in the firstexample, and an IDT having a pair of interdigital transducers 36 a and36 b and a pair of reflectors 37 and 37 formed on the surface of thequartz crystal substrate 35. Electrode pads 38 a to 38 f are provided inthe upper surface of the IC 33. Electrode patterns 39 a to 39 g areformed on the bottom plate 34 a of the package 34. The interdigitaltransducers 36 a and 36 b of the SAW resonator 32 and the electrode pads38 a to 38 f of the IC 33 are electrically connected to thecorresponding electrode patterns 39 a to 39 g by bonding wires 40 and41. The package 34 in which the SAW resonator 32 and the IC 33 is sealedairtight by a lid 42 bonded to the upper part of the package 34.

The SAW oscillator 31 of this example includes the SAW resonator of thisembodiment, and has an excellent frequency-temperature characteristicwith a very small frequency fluctuation in a wide operation temperaturerange and a high Q value. Therefore, it is possible to perform a stableoscillation operation and to realize reduction in power consumptionbecause of low impedance. As a result, a SAW oscillator is obtainedwhich copes with high-frequency and high-precision performance based onrecent high-speed information communication, and includes anenvironment-resistant characteristic such that, even when a temperaturevaries extremely, a stable operation is ensured for a long period.

The invention is not limited to the foregoing examples, and variousmodifications or changes may be made within the technical scope. Forexample, although the SAW resonator of the first example has thereflectors on both sides of the IDT, the invention can also be appliedto a SAW resonator with no reflectors. Although in the foregoingexamples, a quartz crystal substrate is used as a piezoelectricsubstrate, even when a piezoelectric substrate made of a piezoelectricmaterial other than quartz crystal is used, the same functional effectsare obtained. With regard to the electrode structure of the IDT, inaddition to the foregoing examples, various known configurations may bemade.

The invention may also be applied to a SAW device other than theabove-described SAW resonator and SAW oscillator. The SAW device of thisembodiment may also be widely applied to various electronic apparatuses,such as a mobile phone, a hard disk, a personal computer, a receivertuner of BS and CS broadcasts, various processing apparatuses for ahigh-frequency signal or an optical signal which propagates through acoaxial cable or an optical cable, a server network apparatus whichrequires high-frequency and high-precision clock (low jitter and lowphase noise) in a wide temperature range, and a wireless communicationapparatus, various module apparatuses, and various sensor apparatuses,such as a pressure sensor, an acceleration sensor, and a rotation speedsensor.

The entire disclosure of Japanese Patent Application No. 2010-201752,filed Sep. 9, 2010 is expressly incorporated by reference herein.

The invention claimed is:
 1. A surface acoustic wave device comprising:a piezoelectric substrate having Euler angles (−1.5°≦φ≦1.5°,117°≦θ≦142°, |ψ|≠90°×n (where n=0, 1, 2, 3)); and an IDT which has aplurality of electrode fingers on the principal surface of thepiezoelectric substrate and excites a Rayleigh wave in a stopband upperend mode, wherein inter-electrode-finger grooves are recessed in thesurface of the piezoelectric substrate between adjacent electrodefingers of the IDT, the wavelength λ of the Rayleigh wave and the depthG of the inter-electrode-finger grooves satisfy the relationship0.01λ≦G≦0.07λ, the line occupancy η of the IDT and the depth G of theinter-electrode-finger grooves satisfy the following relationships, and−2.0000×G/λ+0.7200≦η≦−2.5000×G/λ+0.7775 where 0.0100λ≦G≦0.0500λ−3.5898×G/λ+0.7995≦η≦−2.5000×G/λ+0.7775 where 0.0500λ≦G≦0.0695λ thethickness H of the electrode fingers and the line occupancy η of the IDTsatisfy any one of the following relationships: (1) 0.010λ≦H≦0.015λ, and0.575≦η≦0.741 (2) 0.015λ≦H≦0.020λ and 0.605≦η≦0.744 (3) 0.020λ≦H≦0.025λand 0.605≦η≦0.745 (4) 0.025λ≦H≦0.030λ and 0.575≦η≦0.745.
 2. The surfaceacoustic wave device according to claim 1, wherein the Euler angle ψ ofthe piezoelectric substrate is in a range of 42.79°≦|ψ|≦49.57°.
 3. Thesurface acoustic wave device according to claim 1, wherein the Eulerangle ψ of the piezoelectric substrate is |ψ|=45°.
 4. The surfaceacoustic wave device according to claim 1, wherein, in an orthogonalcoordinate in which the vertical axis is the line occupancy η (%) andthe horizontal axis is the depth G (% λ) of the inter-electrode-fingergrooves, the line occupancy η of the IDT is in the following ranges, (1)a range surrounded by a polygonal which connects points (0.74,1),(0.75,2), (0.74,3), (0.735,4), (0.741,5), (0.73,6), (0.73,7), (0.575,7),(0.615,6), (0.64,5), (0.66,4), (0.675,3), (0.703,2), and (0.725,1) whenthe electrode finger thickness H is 1.0 to 1.5% λ (2) a range surroundedby a polygonal which connects points (0.702,1), (0.718,2), (0.744,3),(0.731,4), (0.725,5), (0.73,6), (0.745,7), (0.605,7), (0.605,6),(0.63,5), (0.655,4), (0.675,3), (0.7,2), and (0.687,1) when theelectrode finger thickness H is 1.5 to 2.0% λ (3) a range surrounded bya polygonal which connects points (0.718,2), (0.744,3), (0.731,4),(0.725,5), (0.73,6), (0.745,7), (0.605,7), (0.605,6), (0.62,5),(0.645,4), (0.669,3), and (0.695,2) when the electrode finger thicknessH is 2.0 to 2.5% λ (4) a range surrounded by a polygonal which connectspoints (0.745,1), (0.74,2), (0.74,3), (0.745,4), (0.725,5), (0.725,6),(0.725,7), (0.575,7), (0.575,6), (0.605,5), (0.635,4), (0.66,3),(0.695,2), and (0.715,1) when the electrode finger thickness H is 2.5 to3.0% λ.
 5. The surface acoustic wave device according to claim 1,wherein the line occupancy η of the IDT satisfies the followingexpression,η=−1963.05×(G/λ)³+196.28×(G/λ)²−6.53×(G/λ)−135.99×(H/λ)²+5.817×(H/λ)+0.732−99.99×(G/λ)×(H/λ).
 6. The surface acoustic wave device according toclaim 4, wherein the line occupancy η of the IDT satisfies the followingexpression,η=−1963.05×(G/λ)³+196.28×(G/λ)²−6.53×(G/λ)−135.99×(H/λ)²+5.817×(H/λ)+0.732−99.99×(G/λ)×(H/λ).
 7. The surface acoustic wave device according toclaim 1, wherein the sum of the depth G of the inter-electrode-fingergrooves and the thickness H of the electrode fingers satisfies0.04λ≦G+H.
 8. The surface acoustic wave device according to claim 4,wherein the sum of the depth G of the inter-electrode-finger grooves andthe thickness H of the electrode fingers satisfies 0.0407λ≦G+H.
 9. Thesurface acoustic wave device according to claim 5, wherein the sum ofthe depth G of the inter-electrode-finger grooves and the thickness H ofthe electrode fingers satisfies 0.0407λ≦G+H.
 10. The surface acousticwave device according to claim 6, wherein the sum of the depth G of theinter-electrode-finger grooves and the thickness H of the electrodefingers satisfies 0.0407λ≦G+H.
 11. The surface acoustic wave deviceaccording to claim 1, further comprising: an oscillation circuit whichdrives the IDT.
 12. The surface acoustic wave device according to claim4, further comprising: an oscillation circuit which drives the IDT. 13.An electronic apparatus comprising: the surface acoustic wave deviceaccording to claim
 1. 14. A sensor apparatus comprising: the surfaceacoustic wave device according to claim 1.